Optimal pulse defibrillator

ABSTRACT

The model that is developed in the invention is based upon the pioneering neurophysiological models of Lapicque and Weiss. The present model determines mathematically the optimum pulse duration, d p , for a truncated capacitor-discharge waveform employed for defibrillation. The model comprehends the system time constant, RC, where R is tissue resistance and C is the value of the capacitor being discharged, and also the chronaxie time, d c , defined by Lapicque, which is a characteristic time associated with the heart. The present model and analysis find the optimum pulse duration to be d p  =(0.58)(RC+d c ). Taking the best estimate of the chronaxie value from the literature to be 2.7 ms, permits one to rewrite the optimum pulse duration as d p  =(0.58)RC+1.6 ms. The apparatus makes use of the mathematical definition of optimum pulse duration by storing in the control circuitry of the defibrillation system the actual measured value of the particular capacitor incorporated in the system. The optimized-pulse prescription of this invention can be applied to a monophasic waveform, or to either or both of the phases of a biphasic waveform.

This is a continuation of application Ser. No. 08/835,836, filed Feb.18, 1992, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to defibrillation methods, andmore particularly, to an optimum truncated capacitive-pulse durationthat is based upon the time constants of the system and of the heart.

2. Description of the Prior Art

Defibrillation, or causing the cessation of chaotic and uncoordinatedcontraction of the ventricular myocardium by application of anelectrical direct current and voltage, in its most primitive form, goesback to the last century. [J. L. Prevost and F. Batelli, "Sur QuelquesEffets des Descharges Electrriques sur le Couer des Mammifers", ComptesRendus Hebdomadaires des Seances de L'Acadmie des Sciences, Vol. 129, p.1267, 1899.] Because of the large currents required for defibrillation,large-area electrodes are employed. [A. C. Guyton and J. Satterfield,"Factors Concerned in Defibrillation of the Heart, Particularly Throughthe Unopened Chest", Am. J. of Physiology, Vol. 167, p. 81, 1951.]

For reasons of simplicity and compactness, capacitor-discharge systemsare almost universally used in defibrillation. The discharge of acapacitor C through a resistance R results in a curve of voltage versustime (and hence, of current versus time as well) that is a decliningexponential function, with a characteristic time given by the productRC. But it has also been recognized for some time that thelong-duration, low-amplitude "tail" of the capacitor-discharge pulse isdetrimental [J. C. Schuder, G. A. Rahmoeller, and H. Stoeckle,"Transthoracic Ventricular Defibrillation with Triangular andTrapezoidal Waveforms", Circ. Res., Vol. 19, p. 689, October, 1966; W.A. Tacker, et al., "Optimum Current Duration for Capacitor-dischargeDefibrillation of Canine Ventricles", J. Applied Physiology, Vol. 27, p.480, October, 1969.] The exact reason for this detrimental effect is notknown, although plausible speculations exist, with one possibility beingthat field heterogeneties cause arthythmias in significantly largeregions of the heart. [P. S. Chen, et al., "The Potential Gradient FieldCreated by Epicardial Defibrillation Electrodes in Dogs", Circulation,Vol. 74, p. 626, September, 1986.] A convenient way to eliminate thelow-amplitude "tail" of a capacitor discharge is by switching, which isto say, simply opening the capacitor-load circuit after a predeterminedtime, or else when voltage has fallen to a particular value. For thisreason, the time-truncated capacitor discharge has been extensively usedafter its effectiveness was first demonstrated [J. C. Schuder, et al.,"Transthoracic Ventricular Defibrillation in the Dog with Truncated andUntruncated Exponential Stimuli", IEEE Trans. Biom. Eng., Vol. BNF-18.,p. 410, November 1971.]

The defibrillation effectiveness of time-truncated capacitor dischargescan be convincingly shown by comparing an untruncated waveform and atruncated waveform of equal effectiveness. The full discharge waveform10 of FIG. 1A was generated by charging a 140-NF capacitor to 455 V, foran energy delivery of 30 J. But the truncated waveform 20 shown in FIG.1B was equally effective for defibrillation in spite of having aboutonly half the energy, and a lower initial voltage. This demonstrationwas carried through for the case of dogs using a catheter electrode anda subcutaneous patch [M. Mirowski, et al., "Standby AutomaticDefibrillator", Arch. Int. Med., Vol. 126, p. 158, July, 1970], as wellas with a dual-electrode intraventricular catheter. [J. C. Schuder, etal., "Ventricular Defibrillation in the Dog with a BielectrodeIntravascular Catheter", Arch. Int. Med., Vol. 132, p. 286, August,1973.] The latter electrode arrangement was also used to demonstrate thepoint for the case of man. [M. Mirowski, et al., "Feasibility andEffectiveness of Low-energy Catheter Defibrillation in Man",Circulation, Vol. 47, p. 79, January, 1973.] Such demonstrations thatcompact capacitor-storage systems could be used with effectiveness pavedthe way for implantable defibrillator system.

In spite of the dramatic results obtained with time-truncatedcapacitor-discharge defibrillator systems, the waveform specificationshave not been systematically optimized. For example, some manufacturerssuch as Medtronic (in their PCD product) simply specify pulse duration,although the physician can choose and adjust the value. A typical valuemight be a programmable duration of 6 ms. Other manufacturers such asCardiac Pacemakers (in their Ventak product) specify the relative amountof voltage decline at the time of truncation, with a typical value ofthe decline being 65% of the initial voltage. It has become customary touse the term "tilt" to describe the relative amount of such voltagedecline, expressed either as a decimal fraction or a percentage. Inalgebraic language:

    tilt=(V.sub.initial -V.sub.final)/V.sub.initial            Eq. 1

Both of the systems just cited employ the monophasic waveform. Thismeans that it consists of a single-polarity single pulse, specifically atime-truncated capacitor-discharge waveform like that of FIG. 1B.However, biphasic waveforms are also widely used. In this case capacitordischarge is also used, but instead of truncation, polarity reversal isaccomplished (by switching once more), so that a secondopposite-polarity pulse immediately follows the initial pulse, and isthen itself truncated. The result is illustrated in FIG. 2.

Prior art in waveform specification for biphasic systems is parallel tothat for monophasic systems. Specifically, some systems simply specifyinitial-pulse duration. [Baker, Intermedics, U.S. Pat. No. 4,821,723.]Other systems specify tilt. [Bach, Cardiac Pacemakers, U.S. Pat. No.4,850,537.] The central focus of the present invention is to optimizepulse duration by using the model of this invention, which comprehendsboth the time constant of the system (capacitor and load resistance),and the natural time constant of the heart as explained below.

It is worthwhile to examine specific examples of prior-art waveformspecification. In FIG. 3A is shown the simple pulse-durationspecification, applicable to either monophasic pulses or biphasicinitial pulses. And in FIG. 3B is shown a tilt specification, alsoapplicable to either monophasic pulses or biphasic initial pulses.

The foundation for optimizing the time-truncated waveform is a family ofmathematical neurophysiological models for tissue stimulation going backto the turn of the century, with the first important such model havingbeen developed by Weiss. [G. Weiss, "Sur la Possibilite de RendreComparable entre Eux les Appareils Suivant a l'Excitation Electrique",Arch. Ital. deBiol., Vol. 35, p. 413, 1901.] He employed theballistic-rheotome technique for pulse generation, wherein a rifle shotof known velocity is used to cut two wires in sequence, their spacingbeing set and measured. Cutting the first wire eliminated a short from adc source, causing current to flow through the tissue under test, andcutting the second wire opened the circuit, terminating the pulseapplied. Converting the electrical data into charge delivered by thepulse, Weiss found that the charge Q needed for stimulation was linearlydependent on pulse duration, d_(p). Specifically,

    Q=K.sub.1 +K.sub.2 d.sub.p                                 Eq. 2

Subsequently and similarly, the physiologist L. Lapicque collectedsubstantial amounts of data on the amount of current required for tissuestimulation, using constant-current pulses of various durations. [L.Lapicque, "Definition Experimentelle de l'excitabilite," Proc. Soc.deBiol., Vol. 77, p. 280, 1909.] Lapicque established an empiricalrelationship between the current I and the pulse duration d_(p), havingthe form:

    I=K.sub.1 +(K.sub.2 /dp)                                   Eq. 3

(Note that multiplying this expression through by d_(p) yields anexpression in charge rather than current, identically the equation givenby Weiss. Thus, K₁ =k₁ /d_(p) and K₂ =k₂ d_(p).)

Equation 3 of Lapicque shows that the necessary current and the pulseduration are related by a simple hyperbola, shifted away from the originby the amount of the constant term K₁. Hence the stimulating currentrequired in a pulse of infinite duration is K₁, a current value Lapicquetermed the rheobase. Shortening the pulse required progressively morecurrent, and the pulse length that required a doubling of current forexcitation, or 2K₁, he termed the chronaxie, d_(c). Substituting 2K₁ andd_(c) into Eq. 3 in place of I and d_(p), respectively, yields:

    d.sub.c =K.sub.2 /K.sub.1                                  Eq. 4

For the sake of specific illustration, assume a rheobase current of 3.7amperes, and a chronaxie time of 6 milliseconds. Then a plot of currentstrength required versus the pulse duration that must accompany it is asshown in FIG. 4.

Lapicque's model described cell stimulation, rather than defibrillation,but Bourland demonstrated that defibrillation thresholds in dogs andponies followed the Lapicque model, provided average current is used inthe exercise. [J. D. Bourland, W. Tacker, and L. A. Geddes,"Strength-Duration Curves for Trapezoidal Waveforms of Various Tilts forTranschest Defibrillation in Animals", Med. Instr. Vol. 12, p. 38,1978.] In a companion paper, the same workers showed that averagecurrent is a useful and consistent measure of defibrillationeffectiveness for time-truncated pulses of a fixed duration through asubstantial range of durations, from 2 to 20 milliseconds; in otherwords, so long as the exponential "tail" is eliminated, pulseeffectiveness is not very dependent upon waveform details. [J. D.Bourland, W. Tacker, and L. A. Geddes, "Comparative Efficacy of DampedSine Waves and Square Wave Current for Transchest Defibrillation inAnimals", Med. Instr., Vol. 12, p. 42, 1978].

The defibrillation chronaxie for the heart is usually between 2 ms and 4ms, as can be seen in the chart of FIG. 5. (A journal citation for eachentry is given below the chart.) In this synopsis of published data,chronaxie was inferred from a strength-duration curve such as that ofFIG. 4 when such a curve was provided, and these care labeled "given";in the case labeled "determined", chronaxie was calculated from discretedata provided. In the only other case (6. Geddes, et al.), curves weregiven for waveforms of various tilts, and these were averaged to arriveat 2.8 ms. For the overall chart, 2.7±0.9 ms is the average chronaxievalue.

SUMMARY OF THE INVENTION

The inventors have developed an analytic method for waveformoptimization. It builds upon the models of Lapicque and Weiss, thefinding of Bourland, and the data summarized in FIG. 5. To do this onefirst defines a "sufficiency ratio", the ratio of Bourland's rulingaverage current and the current needed for defibrillation according tothe Lapicque model for a heart of a given K₁, rheobase current, and agiven K₂, a charge. Algebraically, ##EQU1## It is simply the ratio ofBourland's available therapeutic current (or the average currentI_(ave)) to the current required according to the Lapicque formulation.Hence for a ratio of unity, the waveform of average current I_(ave) andduration d_(p) will be able to defibrillate a heart characterized by K₁and K₂.

Multiplying Eq. 5 through by the rheobase current K₁ yields anexpression that of course has dimensions of amperes; noting from Eq. 4that K₁ d_(c) =K₂, makes it possible to eliminate theheart-characterizing quantities K₁ and K₂ from this expression. ##EQU2##Thus, we have here an expression in the two pulse-characterizingquantities I_(ave) and d_(p), but in only one heart-characterizingquantity, d_(p), the chronaxie time. Note that for an infinite pulseduration, this current simply equals the average current I_(ave), butfor a pulse of finite duration, it will be smaller than I_(ave). Thiscurrent, therefore, measures the effectiveness of a particular waveformin defibrillating a particular heart. For this reason, the inventorshave named it the Physiologically effective current, or I_(pe), whichcan then be further manipulated in straightforward fashion. ##EQU3##Note further that I_(pe) would be the same as I_(ave) if one had a zerovalue of chronaxie time, d_(c). In this sense, Eq. 7 constitutes acorrection from actual average current necessitated by the chronaxiephenomenon. Since,

    V.sub.f =V.sub.i e.sup.-dp/Rc                              Eq. 8

it follows that delivered charge=C(V_(i) -V_(f))=CV_(i) (1-e^(-dp/RC))Eq. 9

and hence ##EQU4## It is clear that I_(pe) vanishes at both extremes ofd_(p), so that intermediate extremum must be a maximum, definingexplicitly the optimum waveform that can be achieved by varying pulseduration with a particular average current. To determine this optimumpulse duration, let RC=t, and set ##EQU5## Hence, using the system timeconstant (t=RC) for normalization yields ##EQU6##

Using these definitions,

    (Z+α+1)e.sup.-z -1=0                                 Eq. 15

Next, multiply through by -e^(-z) to obtain the simplified equationwhose root is sought.

    e.sup.z -z-α-1=0                                     Eq. 16

Because the equation is transcendental, it cannot be solved in closeform, so define the function on the left-hand side as f(z) and the firstapproximation for its root as z_(o). The Newton-Raphson method gives anapproximate value for the root as ##EQU7## Experience shows thatwaveforms with a tilt of about 65% are effective, and this correspondsto d_(p) =t, or z₀ =1. Hence an appropriate approximate root is:##EQU8## Denormalization yields: ##EQU9## for the approximate optimumvalue of pulse duration d_(p) as a function of chronaxie d_(c) andsystem time constant t. Carrying through the optimization numericallyshows that this estimate is valid within 0.2% for typical values of R,C, and d_(c). Even for extreme values of these system and heartparameters, the approximate value of optimum duration produces a valuefor the current I_(pe) that is within 2% of the optimum. Since(e-1)=1.72=2, the optimum pulse duration is approximately (and somewhatlarger than) the average of the system's time constant t and the heart'scharacteristic time d_(c). In other words, the optimum pulse duration isa compromise between the two characteristic times involved.

Equation 19 for optimal pulse duration can be rewritten as:

    d.sub.p =(0.58)(RC+d.sub.c)                                Eq. 20

Thus, the optimum pulse duration is most naturally specified as afraction of the sum of two characteristic times: that for the system,RC, and that for the heart, the chronaxie time d_(c). Letting d_(c) =2.7ms, this becomes:

    d.sub.p =(0.58)RC+1.6 ms                                   Eq. 21

Because the sensing of tilt is easy, using tilt as a criterion for pulsetermination is straightforward and convenient. Therefore, we define forpresent purposes a new term, intermediate tilt, to be the ratio (usuallyexpressed as a percentage) of the voltage at some intermediate part ofthe pulse to the initial voltage. Next, convert (0.58)RC intointermediate tilt:

Voltage ratio-(0.58)RC/RC

=1-e⁻⁰.58

    =1-0.56=0.44                                               Eq. 22

Thus,

    intermediate tilt=44%                                      Eq. 23

so that the convenient alternate method for expressing optimum pulseduration is

    d.sub.p ={44% intermediate tilt}=1.6 ms                    Eq. 24

where the symbols { } are taken to mean "time interval for". This newmethod for specifying an optimal pulse is illustrated in FIG. 3C, forcomparison with the prior art method illustrated in FIGS. 3A and 3B.

It is evident that the optimum-duration criterion of the presentinvention could be translated into an equivalent tilt specification ifthe value of R were well-known, stable over time and constant frompatient to patient. But typical resistance variation is from some 25ohms to 100 ohms. Furthermore, variations in capacitance exist, with 10%tolerances being typically encountered. Because the new criterion takesaccount of such variations, it is clearly superior to both a fixed-tiltand a fixed-duration specification. To illustrate this superiority wecalculate physiologically effective current I_(pe) in the face ofresistance variations. The higher the I_(pe), the better the criterion.In FIG. 6 is shown the result of such calculations for the popularfixed-tilt (65%) and fixed-duration (6 ms) criteria, as compared to theoptimum-duration criterion of the present invention. Note that theoptimum-duration criterion yields a higher I_(pe) than either of theprior-art criteria through the full resistance range.

The method of the present invention can be applied in the biphasic caseto the first pulse or phase, to the second phase, or to both. If thefirst phase is chosen for optimum duration, there are a number of otherways to specify the second phase. For example, the second phase can bepermitted to decay to an 80% overall tilt, or 80% charge removal fromthe capacitor. This method has the advantage of delivering fixed energyto the defibrillation path, irrespective of the kinds of variationsdiscussed above. Another method lets the duration of the second pulseequal a fraction of that determined to be optimal for the first. Limitedanimal studies indicate a benefit with respect to having the secondpulse less than or equal to the first in duration. Finally, the secondpulse could be made to meet a fixed-tilt or fixed-durationspecification, just as in the prior-art monophasic cases.

One significant aspect and feature of the present invention is adefibrillation waveform of optimum duration.

Another significant aspect and feature of the present invention is apulse duration that comprehends the system time constant, RC, and theheart's characteristic time, the chronaxie, d_(c).

Still another significant aspect and feature of the present invention isa pulse duration that is approximately an average of the system timeconstant, RC, and the heart's characteristic time, the chronaxie, d_(c).

Still another significant aspect an feature of the present invention isa pulse duration that is equal to the sum of the time intervalcorresponding to an intermediate tilt of 44% and 1.6 ms.

Another significant aspect and feature of the present invention is theprogramming of actual values of R, C, and d_(c) into the controlcircuitry of the defibrillation system.

Still another significant aspect and feature of the present invention isthe use of a defibrillation pulse of optimum duration in a monophasicwaveform.

Yet another significant aspect and feature of the present invention isthe use of a defibrillation pulse of optimum duration in at least onephase of a biphasic waveform.

Another significant aspect and feature of the present invention is theuse of a defibrillation pulse of optimum duration in one phase of abiphasic waveform, with the other phase having an equal duration.

Still another significant aspect and feature of the present invention isthe use of a defibrillation pulse of optimum duration in one phase of abiphasic waveform, with the other phase having a duration fixed byoverall tilt.

Yet another significant aspect and feature of the present invention isan increase of defibrillation effectiveness for a given capacitor volumeand system volume.

Having thus described embodiments of the present invention, it is aprincipal object of the present invention to employ a defibrillationwaveform of optimum duration.

A further object of the present invention is to employ a pulse durationthat comprehends the system time constant, RC, and the heart'scharacteristic time, the chronaxie, d_(c).

A still further object of the present invention is to employ a pulseduration that is approximately an average of the system time constant,RC, and the heart's characteristic time, the chronaxie, d_(c).

Still another object of the present invention is to employ a pulseduration that is a function of actual measurement of the system timeconstant, RC.

Another object of the present invention is to use a pulse duration thatis the sum of a time interval corresponding to an intermediate tilt of44% and a fixed time interval of 1.6 ms.

A further object of the present invention is to employ a defibrillationpulse of optimum duration in at least one phase of a biphasic waveform.

Yet a further object of the present invention is to employ adefibrillation pulse of optimum duration in one phase of a biphasicwaveform, with the other phase having an equal duration.

A still further object of the present invention is to employ adefibrillation pulse of optimum duration in one phase of a biphasicwaveform, with the other phase having a duration fixed by overall tilt.

Yet a further object of the present invention is to achieve an increaseof defibrillation effectiveness for a given capacitor volume and systemvolume.

BRIEF DESCRIPTION 0F THE DRAWINGS

Other objects of the present invention and many of the attendantadvantages of the present invention will be readily appreciated as thesame becomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, in which like reference numerals designate like partsthroughout the figures thereof and wherein:

FIG. 1A illustrates a particular voltage-time waveform of a capacitordischarged through a resistor;

FIG. 1B illustrates a particular voltage-time waveform of atime-truncated pulse, produced by discharging a capacitor through aresistor and terminating the pulse by switching, with this pulsedelivering half the energy of that in FIG. 1A;

FIG. 2 illustrates a biphasic waveform for defibrillation;

FIG. 3A illustrates a monophasic waveform of a particular initialvoltage, specified in terms of pulse duration;

FIG. 3B illustrates a monophasic waveform of a particular initialvoltage, specified in terms of tilt;

FIG. 3C illustrates a monophasic waveform of a particular initialvoltage specified by the method of the present invention;

FIG. 4 illustrates a chart of average current strength required fordefibrillation versus the duration of the pulse of that average current;

FIG. 5 illustrates a chart of chronaxie values drawn from the literatureand a listing of the reference citations; and,

FIG. 6 illustrates a chart of physiologically effective current valuesachieved by three waveform-specifying methods in the face ofload-resistance variations.

FIG. 7 is a flow chart showing the decisional steps of a preferredembodiment of the present invention.

FIG. 8 illustrates a schematic representation of an implantablecardioverter defibrillator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A illustrates a particular voltage-time waveform 10 of a capacitordischarged through a resistor, incorporating a particular initialvoltage 12, and a particular energy 14 delivered from the capacitor tothe resistor.

FIG. 1B illustrates a particular voltage-time waveform 20 of a capacitordischarged through a resistor, incorporating a particular initialvoltage 22, and a particular energy 24 delivered from the capacitor tothe resistor, and also incorporating time-truncation of the pulse,produced by terminating the pulse through switching at the particulartime 26, thus eliminating the tail 28 of the pulse, with this particularpulse delivering half the energy of the particular pulse in FIG. 1A.

FIG. 2 illustrates a biphasic voltage-time waveform 30 fordefibrillation, incorporating a first phase 32 and a second phase 34 ofopposite polarity.

FIG. 3A illustrates a monophasic voltage-time waveform 40 of aparticular initial voltage 42 specified in terms of a specific pulseduration 44 of 4.5 ms.

FIG. 3B illustrates a monophasic voltage-time waveform 50 of aparticular initial voltage 52, with the waveform specified in terms of afinal voltage 54, with the ratio of the voltage decline at truncation tothe initial voltage 52 commonly being described as a tilt percentage 56.

FIG. 3C illustrates a monophasic voltage-time waveform 57 of aparticular initial voltage 59 with the waveform specified in terms of atime interval corresponding to an intermediate tilt 61 of 44%, plus afixed time interval 65 of 1.6 ms.

FIG. 4 illustrates a chart 60 that incorporates a curve 62 of averagecurrent strength required for defibrillation versus the duration of thepulse of that average current.

FIG. 5 illustrates a chart 70 of chronaxie values 72, these data takenout of the literature and drawn from the list 74 of cited references.

FIG. 6 illustrates a chart 80 of physiologically effective currentvalues achieved by two defibrillation waveform-specifying methods 82 and84 of the prior art, and from the waveform-specifying method 86 of thepresent invention, all in the face of variations in load-resistancevalues 88, illustrating the superiority of the method 86 of the presentinvention.

MODE OF OPERATION

The present invention makes use of the mathematical definition ofoptimum pulse duration by storing in the control circuitry of thedefibrillation system the actual value of the particular capacitorincorporated in the system.

It will be recognized that the control circuitry of the defibrillationsystem can be implemented in a variety of ways, as described, forexample, in U.S. Pat. Nos. 4,821,723 and 4,850,357, and is preferrablyimplemented so as to include a microprocessor with associated memory asdescribed in detail, for example, at Cols. 9 and 10 of U.S. Pat. No.4,821,723, or as specific electronic control circuitry as described indetail, for example, at Col. 2 of U.S. Pat. No. 4,850,357.

The optimized-pulse prescription of this invention can be applied to amonophasic waveform, or to either or both of the phases of a biphasicwaveform. In the latter case, when it is applied to a single phase, theother can be specified to have equal duration. Another option specifiesthe opposite phase in a way that produces a particular overall tilt, ordegree of discharging of the capacitor.

Referring now to FIG. 7, the decisional steps of a preferred embodimentof the present invention will now be described. The method of operatinga defibrillator begins by sensing a myocardial dysrhythmia in a humanpatient (steps 90, 92). After the myocardial dysrhythmia is sensed,capacitor energy begins to be delivered to the heart (step 94).Capacitor energy continues to be discharged until the capacitordischarge voltage decays a given percentage from the preselected amountof electrical energy stored in the capacitor (step 96). For example,with a decay of 44%, the time period is 0.58 of the RC time constant asindicated by 44%=1-e⁻⁰.58 (Equations 22-24). Energy delivery continuesfor another fixed time period after reaching the desired voltage decaypercentage (step 98). For example, a fixed time period value of 1.6 msis equal to 0.58 of the heart's assumed chronaxie duration of 2.7 ms(Equations 20, 21).

Next, if a monophasic pulse is sufficient, the delivery of energy fromthe capacitor is halted (steps 100, 102). If a biphasic or multiplephase pulse is required, then voltage polarity is reversed as energy isdischarged (steps 100, 104) until the discharge is complete (step 106).

Referring now to FIG. 8, a schematic illustration of an implantablecardioverter defibrillator (ICD) 91 will now be described. ICD 91 isillustrated having a housing 93 and a pair of electrodes 95. Theelectrodes are adapted for implantation in a human patient as is housing93. Housing 93 contains a wave form generating capacitor means 95 forstoring an electrical charge and a capacitor charging means 97 forcharging capacitor means 95. An output switch 99 is provided toselectively discharge the electrical charge in capacitive means 95. Asensing and control circuit 101 is provided to sense cardiac dysrhythmiafrom a human patient and to provide a control signal to switch means 99.

Various modifications can be made to the present invention withoutdeparting from the apparent scope hereof.

We claim:
 1. An improved implantable defibrillator apparatus forproducing a capacitive-discharge defibrillation waveform to be deliveredthrough at least two electrodes adapted for implantation in a humanpatient, the implantable defibrillator apparatus including aself-contained human implantable housing having electrical connectionsto at least two electrodes adapted for implantation in the humanpatient, the implantable housing containing a waveform-generatingcapacitor means for storing an electrical charge, means for charging thewaveform-generating capacitor means, and means for selectivelydischarging the electrical charge in the capacitor means through the atleast two electrodes in response to a control signal from a means forsensing of a cardiac dysrhythmia in the human patient, the improvementcomprising:the means for selectively discharging including means forcontrolling the discharge of the capacitor means through the at leasttwo electrodes to produce a defibrillation waveform comprising at leastone electrical pulse having a predetermined optimum pulse duration basedon an optimization of an effective current of the defibrillationwaveform for a preselected amount of electrical energy stored in thecapacitor means.
 2. The apparatus of claim 1 wherein the optimum pulseduration is automatically set by the apparatus to be the sum of:a firstvalue derived from a fixed time period; and a second value derived fromthe time required for a truncated discharge of the capacitor means ascontrolled by the means for selectively discharging to decay by aparticular percentage of the preselected amount of electrical energystored in the capacitor means.
 3. The apparatus of claim 2 wherein thefirst value is between 1.5 ms and 1.7 ms.
 4. The apparatus of claim 2wherein the first value is 1.6 ms.
 5. The apparatus of claim 2 whereinthe percentage decay is between 40% and 48%.
 6. The apparatus of claim 2wherein the percentage decay is 44%.
 7. The apparatus of claim 1 whereinthe optimum pulse duration is automatically set by the apparatus to beapproximately the average of:a first time constant that is a system timeconstant, RC, of the apparatus, where R is a tissue resistance value ofthe myocardium of the human patient and C is an effective capacitancevalue of the capacitor means, and a second time constant that is anaverage defibrillation chronaxie characteristic time, d_(c) of themyocardium of the human patient.
 8. The apparatus of claim 1 wherein theoptimum pulse duration is automatically set by the apparatus to be thesum of;a first time constant that is a system time constant, RC, of theapparatus, where R is an average tissue resistance of the myocardium ofthe human patient and C is an effective capacitance of the capacitormeans, and a second time constant that is an average defibrillationchronaxie characteristic time, d_(c), of the myocardium of the humanpatient, said sum being multiplied by a small factor.
 9. The apparatusof claim 8 wherein the small factor is between 0.5 and 0.65.
 10. Theapparatus of claim 8 wherein the small factor is 0.58.
 11. The apparatusof claim 1 wherein the optimal pulse duration is determined by the meansfor selectively discharging for a first phase of a multiple phase pulse,said first phase being of a consistent polarity.
 12. The apparatus ofclaim 1 wherein the defibrillation waveform includes a first pulse and asecond pulse of opposite polarity to the first pulse and wherein theoptimal pulse duration is determined by the means for selectivelydischarging for the first pulse of the defibrillation waveform.
 13. Theapparatus of claim 12 wherein the optimal pulse duration of the secondpulse is determined by the means for selectively discharging to be asecond pulse duration value derived from a predetermined total decaypercentage of the preselected amount of electrical energy stored in thecapacitor less the percentage decay at the start of the second pulse.14. The apparatus of claim 13 wherein the total decay percentage isbetween 70% to 90%.
 15. The apparatus of claim 14 wherein the totaldecay percentage is 80%.
 16. The apparatus of claim 12 wherein theoptimal pulse duration of the second pulse is determined by the meansfor selectively discharging to be slightly smaller than the optimalpulse duration of the first pulse.
 17. The apparatus of claim 12 whereinthe optimal pulse duration of the second pulse is determined by themeans for selectively discharging to be a second pulse duration valuesuch that a total duration value comprising the sum of the optimal pulsedurations of the first and second pulses equals a predetermined absolutetime interval.
 18. The apparatus of claim 12 wherein the optimal pulseduration of the second pulse is determined by the means for selectivelydischarging to be a second pulse duration value derived from apredetermined total tilt value for the defibrillation waveform stored bythe means for selectively discharging less the tilt percentage of thefirst pulse.
 19. The apparatus of claim 18 wherein the total tilt valueis between 70% and 90%.
 20. The apparatus of claim 18 wherein the totaltilt value is 80%.
 21. The apparatus of claim 12 wherein the optimalpulse duration of the second pulse is determined by the means forselectively discharging to be a predetermined absolute time interval.22. The apparatus of claim 12 wherein the optimal pulse duration of thesecond pulse is determined by the means for selectively discharging tobe a second pulse duration value equal to the time to achieve apredetermined tilt percentage for the second pulse.
 23. An improvedimplantable defibrillator apparatus for producing a capacitive-dischargedefibrillation waveform to be delivered through at least two electrodesadapted for implantation in a human patient, the implantabledefibrillator apparatus including a self-contained human implantablehousing having electrical connections to at least two electrodes adaptedfor implantation in the human patient, the implantable housingcontaining a waveform-generating capacitor means for storing anelectrical charge, means for charging the waveform-generating capacitormeans, and means for selectively discharging the electrical charge inthe capacitor means through the at least two electrodes in response to ameans for sensing of a cardiac dysrhythmia in the human patient, theimprovement comprising:the means for selectively discharging includingmeans for controlling the discharge of the capacitor means through theat least two electrodes to produce a defibrillation waveform comprisingat least one electrical pulse having a predetermined optimum pulseduration based on an optimal system time constant RC, of the apparatus,where R is an average tissue resistance of the myocardium of the humanpatient and C is an effective capacitance of the capacitor means.